1. Field of the Invention
The present invention relates to compact wide-angle zoom lenses suited to electronic still cameras, lens-shutter cameras, or video cameras and, more particularly, to zoom lenses which have their maximum field angles made wider with a shortening of the total length (the distance from the front vertex to the image plane) for excellent portability and are constructed in a simple form.
2. Description of Related Art
In the field of art of image pickup apparatuses using a CCD or like image pickup element, such as electronic still cameras for shooting still pictures and video cameras for taking motion pictures, the tendency to reduce the camera to ever more compact sizes has prevailed. Along with this tendency, there is a demand for wide-angle zoom lenses of shorter total lengths with the diameter minimized at the maximum angle of field. Of the types of zoom lenses, the one in which the front or first lens unit is of negative refractive power, or the so-called "negative lead" type, is amenable to relatively easy techniques of widening the angle of field. For this reason, the negative lead type has found its use in many wide-angle zoom lenses having a field angle of 70.degree. or wider.
Such an adaptation of zoom lenses to wider angles of field has been proposed in, for example, Japanese Laid-Open Patent Applications No. Hei 2-201310, No. Hei 2-296208, No. Hei 4-235514 and No. Hei 4-235515. The zoom lenses proposed here are constructed, as comprising, in order from the object side, four lens units of negative, positive, negative and positive refractive powers.
Also, in Japanese Patent Publication No. Sho 49-23912 and Japanese Laid-Open Patent Application No. Sho 57-163213, there have been proposed zoom lenses comprising, in order from the object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, a third lens unit of negative refractive power and a fourth lens unit of positive refractive power, wherein, during zooming from the wide-angle end to the telephoto end, the first lens unit axially moves toward the image side, while simultaneously moving the second and fourth lens units toward the object side. The third lens unit is made either stationary during zooming or movable for zooming.
Also, Japanese Laid-Open Patent Application No. Sho 58-4113 has proposed a zoom lens of widened field angles with four lens units of negative, positive, negative and positive refractive powers in order from the object side, wherein all the air separations are made variable to obtain a zoom ratio of 2.5-3 or thereabout.
Also, Japanese laid-Open Patent Application No. Hei 1-216310 has proposed a zoom lens which comprises four lens units of negative, positive, negative and positive refractive powers in order from the object side, wherein, for a zoom ratio of about 2, the total number of lens elements is relatively small.
Japanese Laid-Open Patent Applications No. Hei 5-313066 and No. Hei 8-110470 have proposed another zoom lens which comprises five lens units of negative, negative, positive, negative and positive refractive powers in order from the object side, wherein all the lens units are made movable for zooming.
Japanese Laid-Open Patent Applications No. Sho 59-16248 and No. Hei 6-66008 have proposed yet another zoom lens comprising, in order from the object side, a first lens unit of negative refractive power and a second lens unit of positive refractive power, totaling two lens units, wherein the separation between both the lens units is made variable for zooming, that is, the so-called "short" zoom lens.
Also, Japanese Laid-Open Patent Applications No. Hei 7-52256 has proposed a zoom lens comprising, in order from the object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, wherein zooming from the wide-angle end to the telephoto end is carried out by increasing the separation between the second and third lens units.
U.S. Pat. No. 5,434,710 has proposed a zoom lens comprising, in order from the object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, wherein zooming from the wide-angle end to the telephoto end is carried out by decreasing the separation between the second and third lens units.
Also, Japanese Laid-Open Patent Applications No. Sho 60-31110 has proposed a zoom lens comprising, in order from the object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, and a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein, during zooming from the wide-angle end to the telephoto end, the fourth lens unit remains stationary, and the separation between the second and third lens units is made to decrease.
In general, the negative lead type zoom lenses reduce the total number of lens elements in the entire system for the purpose of simplifying the structure of construction. In addition, the maximum field angle is, for example, 70.degree., the zoom ratio is about 2.5-3, and the F-number for the wide-angle end is about 2.8. With this specification secured, in order to obtain a good optical performance throughout the entire zooming range, a good determination must be made of the refractive power arrangement of all the lens units and the form and the construction of all the constituent lenses. If the rules of design are inappropriate, a large variation of aberrations with zooming results, making it difficult to maintain good stability of high optical performance over the entire zooming range.